This invention relates to novel polylactide compositions e.g., to polymers containing segments of poly(R-lactides) interlocked with segments poly(S-lactides) and to their preparation in various forms.
The optically active enantiomers L-lactic acid (S-lactic acid) and D-lactic acid (R-lactic acid), and the corresponding cyclic diesters thereof, L- and D-(S- and L-)lactides, are known as are methods of polymerizing the enantiomeric acids or, preferably for high molecular weight, their lactides, to the enantiomeric open-chain polymers herein referred to as poly(R-lactide) and poly(S-lactide), respectively, using mainly cationic initiators, e.g. by compounds of tin, antimony, lead, zinc. C. Lavallee et al., Proc. Int. Symp. on Adv. in Polymer Syn., Aug. 26-31, 1984, Plenum 1985, pp 441-461 discuss preparation and properties of racemic and optically active substituted poly(beta-propiolactones). Blends of these poly-R- and poly-S-lactones (1:1) were reported to form a "stereocomplex" having a crystalline melting point of 203.degree. C. as compared to 164.degree. C. for the individual isotactic enantiomers, and a different crystal structure and morphology. Binary mixtures containing an excess of either enantiomer also contained the high-melting phase. The authors describe poly(L-lactide) and poly(D-lactide) as being highly crystalline, melting at about 180.degree. C., whereas poly(D,L-lactide) is amorphous. Blends of the individual poly(lactide) enantiomers were not mentioned. Racemic polylactides, prepared from racemic monomers by these methods, are either amorphous or somewhat crystalline, melting at about 130.degree. to 140.degree. C., while the polymers prepared from pure enantiomeric monomers are optically active, isotactic and crystalline, melting in the range of about 145.degree. to 215.degree. C. Copolymers of the enantiomeric lactides are reportedly crystalline only when over 90% of one enantiomer is present; melting point decreases from about 173.degree. to 124.degree. C. as composition changes from pure enantiomer to 8% comonomer (opposite enantiomer). Polylactide enantiomers are used in various surgical and pharmaceutical applications, including sutures and other prosthetic parts, and as controlled-release encapsulants for biologically active materials such as anticancer agents and other drugs.
B. Kalb et al., Polymer 21, 607 (1980) describe the crystallization behavior of poly(L-lactide) prepared by cationic ring-opening polymerization of the dilactide. The polymer, described as bioabsorbable, biodegradable and biocompatible, was found to have an equilibrium melting point of about 215.degree. C., a Tg of about 55.degree. C. and a viscosity average molecular weight of about 550,000 measured in chloroform. Precipitation of poly(L-lactide) from chloroform solution with a mixture of glycerol and ethanol produced porous fibers having pores of 0.1 to 0.6 micron diameter.
D. L. Wise et al. in "Drug Carriers in Biology and Medicine", Ed. G. Gregoriadis; Acad, Press, N.Y., 237-270 (1979) discusses the polymerization of D- and L-lactic acids and the dilactides thereof, the former providing only low molecular weight polymers. Preparation of high molecular weight polymers from D-, L- and D,L-lactides using organometallic catalysts such as alkyl zinc, aluminum or tin is described. Polymers from the individual enantiomeric lactides are preferred over those from the racemate because sutures prepared therefrom by melt or solution spinning exhibit less shrinkage. Copolymers of dilactide and glycolide and their use in various biomedical applications are also described.
U.S. Pat. No. 4,471,077 discloses that microporous powders can be prepared from a polymer of D,L-lactide acid, D(-)lactide acid, L(+)lactic acid, or a copolymer thereof with another hydroxycarboxylic acid. Porous powder is prepared by cooling a solution of polymer (poly-D-lactide is exemplified) in hot xylene, filtering off the precipitated polymer and vacuumdrying. The exemplified powder had "interconnecting pores", 55% pore volume, and particle sizes largely in the range 100-400 microns. The powders can be formulated with medicants, nutrients, plant growth regulators, fragrances and the like, for controlled dispensation. Although the patent teaches that the polymers can be mixed, no examples or advantages are ascribed to the mixtures in any proportions.
Ring-opening polymerizations of other lactones or heterocyclic monomers, e.g. of beta-propiolactones, alkylene oxides and alkylene sulfides, are known, initiated by ionic or coordination compounds some of which are stereoselective and, in certain cases, avoid racemization of optically active monomers during polymerization. Certain polymers prepared from racemic monomers using stereoselective initiation are reportedly optically active, indicating polymerization of only one enantiomer. D. Grenier et al. J. Poly. Sci. Poly. Phys. Ed., 22,577 (1984); ibid. 19, 1781 (1981); Macromolecules, 16, 302 (1983) disclose the preparation of D- (R+) and L- (S-) enantiomers of poly(alpha-methyl-alpha-ethyl-beta-propiolactone) by ring-opening polymerization of the corresponding enantiomeric, and preparation of the racemic polymer from the racemic lactone. Blends of the polymeric enantiomers were prepared in solution and blend properties were compared with those of the individual polymers. The latter each had a crystalline melting point of about 160.degree. C., while approximately 1:1 (ee equal or less than 0.5) blends all melted at about 202.degree. C. Blends having higher enantiomeric excesses showed two melting points at about 202.degree. and 160.degree. C. respectively. The so-called higher melting complex was shown to have a different morphology and different physical properties to the individual polymeric enantiomers.
K. Hatada et al. Polymer J., 13 (8), 811 (1981) disclose 1:1 blends of R- and S- enantiomers of poly(methylbenzyl methacrylate) which were distinctly crystalline, melting at 228.degree.-230.degree. C.; the individual enantiomeric polymers had little or no crystallinity and liquified below about 160.degree. C.
H. Matsubayashi et al., Macromolecules 10, 996 (1977); P. Dumas et al., Die Makromol. Chem., 156, 55 (1972) disclose preparation of optically active and racemic poly(t-butylethylene sulfide) by polymerization of optically active and racemic monomers, respectively, using a stereospecific initiator. The racemic and active polymers had crystalline melting points of 210.degree. C. and 162.degree. C. respectively, and different crystal structures and morphology.
H. Sakakihara et al., Macromolecules 2, (5), 515 (1969) disclose preparation of racemic and optically active poly(propylene sulfides), the former by sterospecific initiation. X-ray diffraction studies led to the conclusion that the crystal structures of both racemic and optically active polymers were the same.
It is known that the melting points of enantiomers of a given compound are the same and that progressive addition of one enantiomer to the other generally causes a drop in melting point. Usually a minimum (eutectic) melting point is reached, the melting point rising with further addition of the second enantiomer. In some instances, including the classical case of D- and L-tartaric acids, a maximum melt point is reached at approximately the 1:1 composition. This maximum may be higher or lower than that of the individual enantiomers, and in either case is thought to reflect a new crystalline phase ("molecular compound" of the D- and L-forms). In other instances no maximum is obtained. There is no reliable way to predict the behavior of enantiomeric pairs in non-polymers let alone in polymers whose crystalline phases, if any, are more complex.
The art discloses preparation of selected enantiomeric poly(alkylene sulfides), poly(alkylene oxides), poly(methylbenzylmethacrylates), and beta-propiolactones. Poly(methylethylene sulfides) prepared from racemic monomer or from an enantiomer by stereoselective coordination polymerization both melt at about 60.degree. C. but enantiomeric and racemic polymers of t-butylethylene sulfide, prepared with the same catalyst are both crystalline, melting at about 160.degree. and 205.degree. C. respectively. The high-melting racemic polymers reportedly are mixtures of D- and L-enantiomers. Racemic poly(t-butylethylene sulfide) prepared from racemic monomer with ionic catalysts is amorphous. Enantiomers of poly(methylbenzyl methacrylates) prepared from enantiomeric monomers are essentially amorphous, but 1:1 blends of the polymeric enantiomers form a highly crystalline "complex" melting at 228.degree.-230.degree. C.
Ring-opening polymerization of beta-propiolactones, especially beta methyl- or trifluoromethyl beta-propiolactone, has been studied in detail. Coordination polymerization of enantiomeric monomers produces isotactic, enantiomeric polymers melting at 164.degree. C. Blends (1:1) of these enantiomers melt at about 203.degree. C. and differ in crystal morphology and structure from the component polymers. Moreover, the new phase persists in blends containing enantiomeric excesses of as high as 1:45. Formation of a (high melting) complex is reportedly not always the result of mixing isotactic enantiomeric polymers; equimolar mixtures of isotactic enantiomeric polymers of betabutyrolactam, propylene oxide or methylthiirane (methyl ethylene sulfide) show the same thermal properties and crystalline structure as the corresponding individual polymers.
U.S. Pat. No. 3,797,499 (1974) discloses absorbable surgical sutures prepared from poly(L-lactide) or copolymers of L-lactide and glycolide of high tensile strength and hydrolytic behavior and absorbability. The poly-L-enantiomer is preferred because of availability and higher melting point.
D. K. Gilding et al. Polymer 20, 1459 (1979) report the preparation of poly(L-lactide), poly(D,L-lactide) and copolymers of glycolide and lactide using antimony, zinc, lead or tin catalysts, preferably stannous octanoate. Poly(L-lactide) was about 37% crystalline and the poly(D,L-lactide) was amorphous. U.S. Pat. No. 4,279,249 discloses bioabsorbable prosthesis (osteosynthisis) parts preparable from poly-D- or poly-L-lactic acid having enantiomeric purity of over 90%. The latter had a crystalline melting point of 175.degree. C.
U.S. Pat. No. 4,419,340 discloses controlled release of anticancer agents from biodegradable polymers including polymers of L(+)-, D(-)- and D,L-lactic acids and copolymers thereof. U.S. Pat. No. 3,636,956 discloses absorbable sutures prepared from enantiomeric poly(lactides), poly(D,L-lactide) and copolymers. Melting point, tensile strength are reported higher from the individual enantiomeric poly(lactides). D. L. Wise et al., J. Pharm. Pharmac., 30, 686 (1978) describe sustained release of antimalarial drugs from polyL(+)lactide or copolymers thereof with D,L-lactide or glycolide.
The preparation of high molecular weight poly-D- and poly-L-lactides and mixtures thereof in the proportions 1-99 to 99-1, formation of a high-melting phase in the blends, and various medical uses, including surgical thread, artificial ligaments and the like, are disclosed in Japanese Unexamined Application J61/036-321.
As discussed hereinabove, poly(lactides) have many desirable properties for biological applications, but use of even the crystalline enantiomeric poly(lactides) is limited by melting point, hydrolysis rate, sensitivity to solvents, polymeric strength and the like which, while superior to the racemic polylactide, are marginal or inadequate for many applications.
M. Goodman et al., Polymer Letters 5, 515 (1967) describe synthesis of optically active, highly crystalline poly(lactide) from optically pure S(+) lactic acid via the lactide. Solution properties of the polymer dissolved in chloroform, acetonitrile, trifluoroethanol and trifluoroacetic acid were studied.
Fieser & Fieser "Organic Chemistry", 3rd Ed. Reinhold 1956, pp 267-269 describe non-polymeric optically active compounds and the melting behavior of mixtures of opposite enantiomers, including the formation of a "D,L-compound" which may melt higher or lower than the individual enantiomers, depending on their chemical nature, but always higher than the eutectic melting point formed by adding one enantiomer to its opposite enantiomer.